Polyesters are used in a large number of technical fields, in particular in the form of plastic material, from food packaging to the medical field, via clothing and the automobile industry, etc. As an example, certain polyesters (for example polyethylene terephthalate—PET, polylactic acid—PLA, etc.) are used in the manufacture of clothes, carpets, but also in the form of a thermoset resin for the manufacture of packaging or automobile plastics or other parts.
As a consequence, the production of polyester containing plastics has increased dramatically over the last decades. More than 50% of these plastics are used for single-use disposable applications, such as packaging, agricultural films, disposable consumer items or for short-lived products that are discarded within a year of manufacture. Regrettably, plastics may persist for decades depending on local environmental factors, like levels of ultraviolet light exposure, temperature, presence of suitable microorganisms, etc. As a consequence, substantial quantities of plastics are piling up in landfill sites and in natural habitats worldwide, generating increasing environmental problems.
One solution to reduce environmental and economic impacts correlated to the accumulation of plastic is recycling wherein plastic material is mechanically reprocessed to manufacture new products. However, the actual recycling processes use huge amounts of electricity, particularly during the extruding step, and the equipment used is also expensive, leading to high prices which may be non-competitive compared to virgin plastic.
Another potential process for recycling plastic consists of chemical recycling allowing recovering the chemical constituents of the polymer. The resulting monomers may then be used to re-manufacture plastic or to make other synthetic chemicals. However, up to now, such recycling process has only been performed on purified polymers and is not efficient on raw plastic products constituted of a mix of crystallized and amorphous polymers and additives. Moreover, such recycling process is expensive leading to non-competitive monomers compared to virgin monomers.
On the other hand, enzymatic degradation is looked as an ideal waste treatment method because enzymes can accelerate hydrolysis of plastics and can be incorporated into a natural cycle of organic materials. Furthermore, the hydrolysate (i.e., monomers and oligomers) can be recycled as material for polymers. Thus, the depolymerization of polymers contained in a plastic product by enzymes is of great interest, as an alternative to the existing and unsatisfactory processes.
However, this enzymatic approach did not lead so far to the implementation of an effective and industrial enzymatic method of degrading polyester containing material.
Indeed, many bacteria are known to have the ability to degrade polyesters. For instance, regarding polylactic acid, there is a report of degrading enzymes derived from Actinomycetes such as Amycolatopsis sp. (strain K104-1) and from Paenibacillus amylolyticus (strain TB-13). However, up to now, the identified polypeptides have poor degrading ability and allow only degradation of the polymer in emulsion form. There are a limited number of reports on microorganisms capable of degrading polyester-containing material in film or pellet form, and their enzymes are poorly known. Furthermore, most of said identified polypeptides are efficient solely at elevated temperature. Their use thereby increases the cost of a thermal degradation process.
In view of the foregoing, there is a need for novel enzymes suitable for degrading polyesters and more particularly for degrading polyesters contained in plastic products.